(a) Field of the Invention
The present invention relates to a transmission system for digital signals. More specifically, the present invention relates to a pre-equalizer, a vestigial sideband (VSB) transmission system using the pre-equalizer, and a transmission method thereof.
(b) Description of the Related Art
In Korea, the ground wave digital broadcasting system adopted the American VSB system as the standard of the transmission system in 1997, and after experimental broadcasting in 1999, proceeded in earnest with it in the National Capital area in the second half of 2001. One of the most important pieces of equipment for the digital broadcasting transmission is the digital TV (DTV) transmitter and repeater. In the case of conventional analog broadcasting, sub-channels are provided adjacent to the broadcasting channels so as to reduce interference between adjacent channels, and they are not actually used. With the switchover of analog broadcasting to digital broadcasting, however, the channels adjacent to the broadcasting channels can be used to efficiently utilize limited frequency resources.
For this purpose, a channel filter must be provided at the final stage of the DTV transmitter. The reason for this is that the channel filter is used as a band-pass filter for eliminating an unnecessary frequency component from the out-of-band to reduce interference between adjacent channels.
For a channel filter of which the band has a constant magnitude, the filtering characteristics are excellent, but the group delay characteristic is extremely large at both ends of the band, which lowers the signal-to-noise ratio (SNR) of the transmitted signal.
Thus the use of a pre-equalizer for compensating for the channel filter is essential in order to enhance the performance of the transmitter. Without the pre-equalizer used as a compensating means, the SNR of the transmit signal as the final output of the transmitter is lowered and the performance of the transmitter is deteriorated due to the spectrum energy of an undesired band.
The equalizer used for the receiver of the VSB transmission system is usually of a decision-feedback type. This is to solve problems caused by multiple paths between a transmitter and receiver and a distortion of channels. But the use of a linear equalizer rather than the decision-feedback type equalizer is adequate for the VSB transmitter, in which the multiple paths induced by the channel filter are not so long.
Having knowledge of the signal to be transmitted, the conventional pre-equalizer uses the least mean square (LMS) algorithm to calculate a tap coefficient for compensating for a group delay characteristic. However, the structural complexity of the equalizer increases because a means for extracting the transmit signal in real time is necessary in order to use the LMS algorithm.
Moreover, the tap coefficient is updated in real time for the transmit signal with a consequence of an increase in the probability of errors occurring during the signal transmission, so that the tap coefficient cannot be calculated accurately and the SNR of the transmit signal decreases.